Ignore embedded thumbnail art when too large

[averne]

This encode contains a ridiculously large embedded thumbnail art (4785x8529 png, 98.5MiB), which this script ends up passing as inline b64-encoded url. This triggers a crash deep inside glibc which I'm not willing to investigate, since I value my sanity. Instead, just put a threshold on the size for an acceptable thumbnail art. 25MiB is already way bigger than it needs to be, but I verified it working for a ~37MiB thumbnail file.