Silly Q from a D-SLR newb

Hi from a newbie.

Just got the 300D (Black Jessops Kit) and a 75-300 USM [IS]

Very pleased with my first shots, however one thing confuses me.
Being new to D-SLR but not digital how come the image sizes are wierd???

I am used to 800x600, 1600x1200, 2048x1536 etc

The300D sizes are strange to me, 2048x1360 1536x1024

Am I missing something real obvious here?? Is like Medium, 35mm, aps formats etc type thing???

Thanks in advance