I don't know if the old board software did this, but it seems that the new software will re-save an uploaded image for the thumbnail even if the uploaded image is already small enough to fit the thumbnail.

This is most obvious with a small resolution animated PNG, such as the bouncing ball example on Wikipedia's APNG article (the image is only 100x100 pixels). If one uploads said bouncing ball to GUROchan, the thumbnail will not be animated yet clicking the image and viewing the actual uploaded file will result in it animating. This can only happen if the board software is actually creating a new image specifically for the thumbnail regardless of the uploaded image's pixel resolution.