alamy stock photo site wants photographers to upload JPEGs that were of initial
size of at least 17MB. My Rebe's Xti the biggest JPEG file was 10MB and
that was only after conversion from RAW.
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As has been noted, they're referring to TIFF files not JPEGs.
Use a pro-level camera with a “true” (non-interpolated) resolution of at least 6 megapixels. This will give a file size of at least 17MB at 8 bit.
Consider 6 million pixels ("Mp" to some people) with each pixel having 8-bit red, green and blue values. Each 8-bit value is a byte, so 6 * 3 == 18 MB. (Not "Mb" which is used to refer to megabits, commonly used in data communications)
That's when saved as a TIFF: when you save as a JPEG all that data gets squashed down dramatically.
A point of confusion for some people, if you open up a JPEG file from a 6 Mp camera and look in Photoshop's Image Size dialog it will say something like "Pixel Dimensions: 18.0M". That's the number they're referring to, but it's not necessarily the same as the file size.
As for the issue of "non-interpolated" resolution, some cameras (e.g. those with Fuji SuperCCD sensors) have a smaller number of pixels arranged in a non-square pattern, and the camera or RAW converter rearranges these to produce a different number of pixels in the final image. In the current Canon cameras the sensor has a square sensor pattern and this isn't an issue.
I am thinking of getting 40D, but I could not find the information if 40D will pass alamy's requirements as a pro camera.
It does, as does your XTi. At least the number-of-pixels requirement...