Garner's Modern American Usage

You can solve this by allowing NULL in the foreign key column tblImageFlags.resolutionTypeID.

PS Bonus points to whomever tells me whether, in the case of databases, it's "indexes" or "indices".

The plural of index should be indexes.

According to "Modern American Usage" by Bryan A. Garner:

For ordinary purposes, indexes is the preferable plural, not indices. ... Indices, though less pretentious than fora or dogmata, is pretentious nevertheless. Some writers prefer indices in technical contexts, as in mathematics and the sciences. Though not the best plural for index, indices is permissible in the sense "indicators." ... Avoid the singular indice, a back-formation from the plural indices.