What size Photo Art should I put on my brochure?

The photo will be about 2.5 inches x 2.5 inches. The options are 0.5 megapixels (72 dpi - 900x600) or 1600x1066 300dpi 1.7 mpixels or 3504 x 2336 (300 dpi 8.2 megapixels)- I am buying...the higher the resolution, the higher the cost. All are JPEGS.