The following is a partial summary of various options for decorating T-shirts, hoodies, etc. with custom images such as art, photos and graphics (including my latest obsession, QR codes which can enable you to more easily send smart phone owners to your web sites, Amazon aStore sites, blogs, etc.).
HAND PAINTING AND DYEING
Well, OK, this is not really an option for precision reproduction (e.g., the QR codes), but arts and crafts still enjoy decorating their garments with markers and paints designed for use on fabrics.
The old tie-dye method offered interesting options, especially when combined with screen printing. Ditto for batik.
BLOCK PRINTING (Linoleum, Wood, Rubber or Photopolymer Stamps, etc.)
If you've ever made relief prints using linoleum, you should know that block printing has long been an effective way of decorating fabric and fabric items. Difficult if you require multiple colors, but great for line art in single colors.
This would be another option if you want the option of repeatability. Nice as long as you do not mind the look of stencils, which require connecting "bridges" between the islands (although you can circumvent this if you cut multiple stencils based on similar images with different locations for the bridges). Hand cut stencils have always been an option, but they can be a pain, especially in the era of computer graphics. If you can find a company which can use a laser cutter/engraving machine to do a one-off stencil for you, it's worth it.
Google search to find a company which will make such a stencil for you. Even Etsy has a listing for a company which will do so; click this link for information.
Screen printing used to be called "silk screen printing", but the word silk is usually omitted these days because the screens tend to be made of synthetic materials. Needless to say, the main advantage is elimination of the issue with regard to islands and bridges, in terms of a comparison with standard stencils.
Hunt Manufacturing (Speedball) still makes traditional screens, frames, inks and other materials for people who want to try their hand at screen printing. But there are now some modern options in terms of making things easier. Print Gocco (Japanese) was probably the first kit of this kind, but it was eventually driven out of business by digital printing options. Since then, there are several relatively new options: YuDu, the Tulip ScreenIt, and the Plaid Simply Screen kit. (The Plaid is a bit primitive compared with the YuDu and the ScreenIt, but if you take care, you should still be able to use it to get reasonably decent results on a limited quantity basis. The Simply Screen Kit seems to be the most affordable option by far.
By the way, if you want to screen print full color images, you can, provided that you find a company which can make color separations from your image. Resolution is limited by the "mesh count", but it can still look pretty cool, if it's done by a pro. I've seen some beautifully screen printed shirts featuring wildlife (at Bass Pro Shop), motorcycles (most noticeably, Harley Davidson) and other subjects.
By the way, the fine art term used for screen printing is "serigraphy".
One has long been able to buy sheets of transfer materials designed to work with toner-based prints or inkjet prints. (Make sure you have the right kind of sheets.)
Avery sells transfer sheets of this kind in office supply stores. Some even work on dark fabrics (which is cool, because sweat stains tend to be less noticeable under the arm pits on black shirts).
You can transfer such prints with a standard iron, but I highly recommend a professional heat press if you plan to make more than a few shirts this way. Here's a link to a YouTube video showing the use of such a heat press. http://www.heatpressinc.com/ will take you to one company which sells heat press machines of this kind.
Here in Bellingham, there's a little shop known as Graffiti Shirts, where they will transfer images to your shirts using a heat press of that type.
Serigraphy was once the preferred method of reproducing art, but it tends to be more complex that giclee printing with a wide format printer from a company like Epson, Canon or Hewlett Packard. And then there are the wide format digital printers (such as the Durst Rho) which can print almost anything on any rigid substrate (such as wood, glass, Gatorboard, Duho art panel, etc.), even if it requires the use of opaque white inks so that the image will have maximum color fidelity.
But I was discussing shirts and garments. The best digital printer for this type of printing is the Direct To Garment printer. This seems to be the kind of printer used by companies such as CafePress.com and Zazzle.com, for printing digital images on T-shirts. If you are made of money, buy your own DTG printer; otherwise, do the smart thing and have the printing done by a company which does it on a regular basis.
So there you have it: A basic summary of the primary methods of creating images an
d graphics on T-shirts and garments. Of course, I haven't discussed options like embroider from digital files, etc., but those are not the most predominant methods in use today.