Hi
What kind of stamps are they?
Wood mounted/clear/ foam.
Are they very detailed pictures like etchings?
What is the problem with the impression you get? Are you getting ink transfer from the surround of the stamp, missed areas of stamp? Too much ink in between a detailed image?
Speaking personally, I find Versfine Onyx black an excellent ink for detailed stamps.
I use a thin sheet of funky foam under the paper I'm stampin onto for large stamps, to avoid uneven pressure that I may apply.
If it is those white etching style stamps, I've heard Dawn suggest that people trim down the foam until they are close to the image to avoid access ink getting on to the stamp surround.
Hope that is of help? If not if you can give me some more details about the specific problem with the image you stamp, what inks you are using and the kind of stamps we are talking about, I'll have another go

Best wishes
Billie

PS Can you post any pictures of the stamp or the problem images?