You need to apply what's called a high pass filter. If corel photopaint doesn't have this you can get the same effect with gaussian blur. Copy the photo and paste as a new layer. Apply a large gausian blur to the new layer that is larger than the biggest feature you want to preserve. (if your squares are 50 pixels across make your blur 70 pixels wide) Convert the layer to a negative image and make the layer 50% transparent. Merge the two layers and this will flatten out the shine from the flash on the top of the image. It will also flatten out the tone and contrast and make the image look flat. Crank up the contrast, tone, and color saturation until the image looks normal again. Now the brightness will be uniform across your image so you can tile it. You should do this before cropping the image to make it tilable.