I recently found out (to my horror) that sandbox solutions do not support (among many other things) the publishing field types. For example, if you want to use the "LinkFieldValue" class from the Microsoft.SharePoint.Publishing.Fields namespace in a sandbox solution, then you are out of luck. So far, fair enough - annoying, but fair. I figured that I can overcome this by not using that class to parse the details of the image from publishing pages, and I will write my own function that will tease out the image URL from the string that the field returns.
How naive am I!
When I tried running this: imageUrl = item[field.Internalname);
The code would time out, and not return anything. The entire page would just hang. No exception thrown or any reason given.
My solution was to use: imageUrl = item.GetFormattedValue(field.InternalName);
Apparently this gets the string value without trying to go through anything that is not supported in sandboxed solutions.
Update: after a bit of more trial and error I found that the exception thrown (sometimes) is that the assembly (the publishing one) is not serialiazble:
'item[this.ImgFieldName]' threw an exception of type 'System.Runtime.Serialization.SerializationException'