Well, the thing you are telling about, is called "accessibility compliance". We were thinking a lot about it in the past, but suspended it for a while. Accessibility Compliance is a very difficult question, and according to W3C standarts, even "A" level (there are A, AA, AAA) is difficult to achieve.
miniBB has only HTML compliance. And it has no even A level. So if we decide to implement it, it will be much harder, than only to specify ALTs.
But probably, it's a good suggestion... and we could even implement this in the next release. At last what is depending on ALTs... thanks!