When a pic size is 2500 pixels, what does that mean?

Is it about the diagonal or something?

A picture can be 2500 x 3750 at 300 ppi (or 72 ppi when posted on a website) or 2500 x 2500 pixels at 300 ppi or even 2500 x 1667 pixels at some ppi number. These are all samples of possible image sizes.

My cameras have 12 mp sensors so produce 4288 x 2848 pixel image files.

When I use the studios Nikon D800e, it produces 7360 x 4912 pixel image files.

As you may begin to see, the size image is based first upon the number of pixels on the cameras sensor and what resolution the final image is when you have finished retouching or cropping an image.

I usually save my JPEG images that are bound to be posted on an internet website to a reduced resolution image file that is 600 x 400 pixels at 72 ppi

Here is a sample of such an image

.html? Sort=3&o=19

If you look at the EXIF data on the image, you will see that it was reduced to a 600 x 399 pixels at 72 ppi image file that takes up under 220 kb in size.

The original image file is a RAW file that is 15 mb and 4288 x 2848 pixels at 300 ppi

I have never seen digital images referred to by diagonal size. 2500 pixels is tiny; about the size of the avatar image displayed next to this answer. Depending on the context, it might refer to one of the dimensions of the image, like width.