That's a BIG topic
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I'll try to keep this short and sweet
The first thing you need to ask is the kind of images you want to make or alter. Broadly, there are two kinds of computer images in the world, raster and vector. Both have their places in the world, and you may want different programs that are good at each job.
A raster image is your basic block of pixels, this kind of thing is suitable for photos, Web graphics, screen shots, etc. It's not ideal for things that you want to send to a printer (by printer, I mean the shops with presses, not so much something you'll be printing at your desk). Of course, photos and the like do end up in print, but drawings and such don't do as well in this environment (the jaggy lines thing I mentioned earlier, and other not-so-fun problems).
The 800-pound gorilla for working on raster images is Adobe Photoshop. It can do a lot, but it's a high-maintenance relationship. It gives you a huge bag of nifty tools but little in the way of guidance on how to use them well. There are all kinds of training options, ranging from traditional classes to videos to books to tutorials and "recipes" on the net, that can help you to become proficient.
If you're new to this, there is a friendlier edition called Photoshop Elements. It has more more step-by-step help built in, and lots of example effects that you can use as-is or modify to your liking. It's missing some of the tools in the high-end Photoshop, such as the modes you would need to make the image useful to those print shops, some of the geekier filters, and some of the more fiddly settings for type like tracking/kerning/leading/etc. (it will, however, happily do the text stretching thing you asked about at the start of this thread).
There are some vector drawing capabilities built into Photoshop, but they are limited, not much of a help if you ultimately need vector output.
If you're not sure you want to pour money into this kind of program just yet, there are free alternatives. Gimp and GIMPshop and nearly full-featured, and a much simpler cousin called Seashore may be worth a look.
There is also GraphicConverter, which does much more than conversions. Its painting/drawing capabilities are rather primitive, but it has lots of basics like filters and text placement that can be very useful.
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Vector images are more suitable for things like logos and diagrams or line drawings. Their big advantage is that they are made up of lines and shapes that can be scaled as large as you need without losing detail or getting jaggy. Vectors are not well suited to photos, and currently Web browsers have rather poor support for them.
The most common formats for vector images are Adobe PostScript and its close cousin PDF. An up-and-coming format called SVG should, in the future, allow Web browsers to easily display this stuff on regular web pages, but for now a vector drawing can still be exported to a raster image for the Web (in any size you need).
For vector drawings, the big pig is Adobe Illustrator (yes, everything graphical seems to be owned by Adobe these days). PostScript is pretty much its native language, and it does its job well. It's also even more intimidating than Photoshop if you're new to this.
A much simpler alternative (and free!) to start out with might be Inkscape. Its native format is SVG, which will give you compatibility with Illustrator, and it can also save to the more Mac-friendly PostScript.
You may want to give the OmniGraffle trial a spin too. It's really intended for charts and diagrams, but the current version has tools like bezier curves that are staples of more conventional vector drawing programs. This may be a good starting point if you're not sure where to begin but want to learn basics that you can still use in the more traditional alternatives.
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There are many more programs where these came from, but 85 paragraphs ago I promised to keep this short and sweet
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