123rf.com
Stock photos:- Images should be at least 4 Million pixels (2200 x 1700). Anything lesser will be automatically rejected. JPEG format only with minimal 80% quality compression.
Vector illustrations:- Vector image file in EPS format. Each file must not exceed 5MB. Attach a RGB JPEG preview of your image at 1600 x 1600 pixels or larger.
Shutterstock
Stock photos:- You may upload photos in either JPEG or TIF format. The minimum size of uploaded images is 4.0MP. For example: a photo which is 2400x1800 adds up to a total of 4.32 million pixels.
Vector illustrations:- upload two files for each vector illustration you want to submit. One file is the vector in EPS format (.eps extension) The second file is a JPEG preview of that vector. The preview should be approximately 400x500 pixels. Each file should be named the same - except for the extension. (e.g. vector_example.jpg, vector_example.eps). Please keep your vector files under 5 megabytes and be sure to save them for compatibility with Illustrator 8. Also please note that Illustrations go only in the Illustration category.
fotolia.com
Stock photos:- Image must be in JPEG format. Resolution must be greater than or equal to 2400 x 1600 (4 Million pixels) & file must be less than 20 Megabytes.
Vector illustrations:- Vectors illustration in SVG format is accepted.
iStockphoto.com
Stock photos:- iStock accepts RGB, JPG files only. All files must be 1600 x 1200 pixels or larger.
Vector illustrations:- When uploading Vector files, one (max) 270x270 preview/thumbnail JPG is required. The source file for Vector/Illustration files must be a zipped EPS or AI file.