Is there any way to auto-generate a 150x150 thumbnail image for each post based on the post's selected featured image? This would be preferable as opposed to manually going into each post, copying the images 150x150 image URL and pasting it thumb custom field.
Inside your post loop you can do;
This first checks for the existence of a featured image and if so it will display one at your default 'thumbnail' size as set in your settings/media options.
This would also work within your loop;
This would work (but can be used outside of your loop);
Beyond that there are other functions to help you customize sizes should that not achieve what you want. However the above defaults to 150x150.
Let me know how you go.