Well, there's only so much "real estate" on the canvas.
When making a collage via this method, you have to do the math, and crop the pictures so that they will fit. They may be in portrait and/or landscape orientation, so that has to be taken into account.
Be sure to have the resolution of the canvas and of the pictures the same value.
For printing, it is desirable to have the resolution in the 240-300 px/in range, for web work 72 px/in is ok.