Where is a good place to find scientific images w/o violating copyright?

[Simfish]

Okay, so I'm thinking about creating a website about astronomy. One thing I'd like to do is to include a lot of figures from journal articles. But I fear that including these diagrams will violate copyright. Are there certain journals that pretty much allow anyone to post figures without violating copyright? I know that Wiki is a good place to start (as all the images there are under the creative commons license), but I'd like to highlight examples/figures that aren't on Wiki, since my website will be useless if it just gets everything from Wikipedia.

[MATLABdude]

You could ask the publisher for permission to use the images, but I don't know how many of them would appreciate high-volume use on a public website. Alternately, if these are space pictures that you're looking for, many come from NASA / ESA anyways, and, IIRC, are public domain free for use (so long as attribution to NASA / ESA is maintained).
http://en.wikipedia.org/wiki/File:Messier51_sRGB.jpg

[Simfish]

Oh okay. Yeah, I don't really care much for Hubble/space pictures (there are already enough of those sites); I care more about diagrams that actually explain the physical processes involved.