Search engines can read and analyze only text and not images. I agree, the images make your resources more attractive to the readers. But unless people read it, there is no use for it. To make people read your resources, we need to get them listed in search engines. To get your resources listed in search engines, we need to have a lot of text description.
When you add resources, please focus on text and not on images. Add lot of description and include the keywords few times in a meaningful manner.
Do not embed images taken from other sources. They are copy right protected to the original owner who made it, even if it is not written that it is copy right protected.
You may attach images to a resource if the image is designed and made by you.