The issue heres comes from the fact that dynamic types (module builder items) and content types (blogs, news, products as of 6.0, etc) are different. While dynamic types have support for Guid and Guid fields, content types currently do not. We are always looking to improve the product and one of the ways we hope to improve soon is to merge these types so that Guid and Guid are available to all types.
In the meantime you have two options. One is to add a string field and create a field control that uses the string value to store the image url or guid and interpret it back into the actual url or ID when selecting, removing, updating, etc. The other option is to migrate that content to a dynamic module builder based module created in the backend. Easy to do with the point and click creation but you lose some functionality of content types such as granular permissions per object and direct out-of-the-box linking to the alternative publishing methods.
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system and vote to affect the priority of the items