This is indeed currently not supported in TWebMyCloudData or TWebMyCloudDbClientDataset. However this is a good suggestion and we'll consider adding filter options in the future.
As a possible alternative you could try using the TTMSFNCMyCloudData component from the TMS FNC Cloud Pack which does support adding filters.
TTMSFNCMyCloudData seems indeed to work fine for my needs. I even was able to download and show an Image that is stored in a Blob field, But nothing I tried with uploading pictures works.
I found entity.Blob['PICTURE'].LoadFromFile(f) but do not know how to deal with TMSFNCUtilsFile.