<img onclick="displayURL()" title="Display URL" ../>
<input type="image" alt="Display page URL - popup window" onclick="displayURL()" src=".." title=".." />
Context: the page this is taken from is typically displayed 'embedded' in an
displayURL displays a popup message box that shows the URL for the regular page on the originating site.
Changes: the image was changed to an image-button, and an ALT attribute was added to match the title.
Taken from: a real example from a research web site hosted at The Open University (I'll save their blushes!)
So, the thinking behind this, is that for people with a disability and using assistive technology (for example a screen reader for those with visual impairments), semantics matter. A button containing an image says "press me", an image on it's own probably won't.
These are simple (for a developer), concrete examples of what I hope are good and bad practice for Web accessibility. Sometimes they may seem trivial to you – probably because you have some experience with accessibility. I hope they won't be trivial for everyone. Comments welcome!
[ Added 'context' and 'changes' following Tim's comment. ]