Picture Quality

File size

One big difference from album sites etc is that the Stock Photo agencies require a minimum file size instead of the a maxumum file size common on most album sites/community sites. Common sizes was before 2Mpx i.e. 1600x1200 but are now 4 Mpx (about 2400x1600). You are not allowed to interpolate smaller sizes as it just adds filesize without adding any details/quality. Not entirely rational because on some sites they uppsize you images.

You will soon learn a few enemies:

CA - Chromatic aberration

This is a fault in your lens typically visible on wide angles with full aperture in high contrast areas. This is your big enemy on sites like istockphoto or fotolia, especially if your lenses are cheap to average. Learn how to fix this with software, Capture NX does it automatically and Lightroom/Photoshop with some minor work.

Artifacting

Artifacting is defect caused by compression or filtering in the camera and/or in your processing software. Try save a jpeg in low quality/hight compression and you will recognise the effect. Sharpening, increasing contrast, adjusting curves, CA-reduction all cause artifacting. To get rid of it you need to make sure your pictures are stored with the highest quality in the camera, do not increase contrast or colour too much in camera or in your software. Or run a "cleaner" like Jpeg Enhancer with drawbacks like loss of some details.

Blur

Don't add any, leave it for the buyer.

Focus

If your focus is bad you're gone. If you put focus in an unusual spot, make sure to notify the reviewer.

DOF

Shallow DOF (Deph of focus) looks nice right? Think twice, you are selling usable pictures, not arty (see below).

Arty picures

Generally, arty pictures do sell. If you have a message with the picture. Otherwise, leave them for your nice photowall.