i am running forum 5.02
I cannot upgrage to dnn 6. as it is not yet compatible with NB Store......
and so i cannot upgrade forum to 5.03....

I believe you're running DNN v5.6.3 (looking at the error message in post 1 of this thread). In light of this, I saw this in the Release Notes on DNN Forum v5.0.2.

Release Notes
The same module as 5.0.1, except compiled against DotNetNuke 5.6.3 (and therefore is a minimum requirement) and the same version of Telerik (2011 SP2) included with it.
NOTE: This was compiled against, and for, 5.6.3 and is included with that release (Not released yet, as of June 12th).

http://dnnforum.codeplex.com/releases/67840/download/247618

If you've tried repairing / reinstalling the package and there's still an issue... Then, I'm sorry, I don't know what else to suggest.

I just ran into the same problem. I am running Forum 5.0.3 on DNN 7.0.3.

Looking at the source code for Forum software, it appears that "AvatarProfilePropName" property returns an empty string that the caller is trying to convert to an integer value.

For those profile photos whose size is less than 4K, we don't run into this problem. My suspicion is that profile photos with more than 4K size are being stored differently.

Peter