Looking at the log file that you sent me and the messages that you forwarded, it’s clear that SpamSieve did not miss those messages. Rather, it was never asked to determine whether they were good or spam.
You can see this because whenever SpamSieve examines an incoming message, it adds a “Predicted: Good” or “Predicted: Spam” entry to the log. The messages that you forwarded have no such entries in the log; they only say “Trained: Spam (Manual)”, which means that you told SpamSieve they were spam. Incidentally, this is why the “filtered messages” statistic that you reported seems low; not all of the incoming messages were filtered.
Aside: Please do not forward spam messages to me. Instead, enable the Save false negatives to disk preference, so that SpamSieve saves the spam messages that it didn’t catch to disk. Then, send the false negative files to me along with the log. This is better for two reasons. First, if there are no false negative files (or not as many as you expected), this is an easy way to see that the messages were not filtered in the first place (thus pointing to a setup problem). Second, if SpamSieve did filter a message and classified it incorrectly, then I will want to analyze the original message contents, not the modified version that your mail program generates when you use the Forward (or Redirect) command.
In order to get SpamSieve to catch these image spams, you need to make sure that your mail program asks SpamSieve to filter them. Thus, please check the setup to make sure that the SpamSieve rule is at the top of the list (so that other rules don’t prevent it from being applied) and that the rule conditions cause it to be applied to every message. For Apple Mail, the rule should look like the one shown here.