Images don't show when using custom size

I thought the default thumbnail size of 100 pixels was too small and the small size of 240 pixels too large. I decided to choose a custom size of 150 pixels to fix this, but then the images would no longer display:

http://alexandervanloon.nl/afg_customsize.png

As soon as I switched back to the default (non-custom) sizes the images appeared again, so there seems to be a bug here. I did not select the option to crop to a square.

http://wordpress.org/plugins/awesome-flickr-gallery-plugin/