The graphic designer I use sends me jpg files that I often need to crop or resize. I find that the quality of the cropped or resized version is always worse than what I received in the first place.
What is the reason for this, and how can I get around the problem?
jpeg is a lossy format. by definition, the quality will always decrease with every re-encode (separate open and save).
ways to get around:
* use a non-lossy file format, such as png.
* try using as little compression as possible when saving as jpeg. In Seashore, you can set this in the Save dialog, under Options. Move the compression slider further to the right (higher percentage).
I can see the drop-off in quality, even before saving the file (i.e. and having the option of selecting the compression). The same files re-sized in iPhoto remained pristine. The problem with the latter option, though, is that it's easy to crop, but I haven't found a way of resizing to a particular size in pixels.
Ah OK, I'm with you; the scaling algorithm isn't up to scratch. There are a couple of options in the resize dialog, such as interpolation, have you played with those at all?
Another option for you, is to use Preview. Under Tools -> Adjust Size. That can do by pixel.