You can create your own transaction with the image of your choice as attachment or you can test it using existing transactions with file attachments

Existing transactions with file attachments:
Please be aware for security we can only render https links.
8j4xiy9S9czzL2yeAGXmZfWJ67acy4PFSKJgvmhMRKGT (image)
FxZhKsTXstB3uaUv8yDpxeguKr5GZwRBxgiSyWhSYUAk (image)
2k7FAxUAgsXDmd7MF6KQrbarS6fpW9qgJtvBcYpq9i4a (pdf)