How can I fix these errors?



Well all the different block types listed there each require their own images so if you change the block size you have to choose new image files for them so they match the new size you want. You should be able to create new image files yourself or resize existing ones, but they may not look right if they are resized. But if you look at /admin/block.png you can see an example of how a regular empty block looks. Other blocks may be just solid squares of colour but the blank default ones have lines on 2 of the sides.