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Once the trial expires Photoshop will not work anymore, however it seems that Adobe didn't remember ImageReady, so you can keep using ImageReady as long as you want(just don't uninstall Photoshop)
ImageReady is pretty good for designing buttons and Optimizing web images. You can do gif images, export to a variety of formats, including gif, png, jpg, swf, and others.
Tip: to make it save as png or jpg by default, instead of the gif. Go to Edit > Preferences > Optimization and on the group box click "Named Setting:" and select your format of choice.