Thermally developable photographic elements

A photothermographic imaging element comprising a substrate having on at least one side thereof a layer comprising a photographic silver halide in reactive association with a silver source material, a reducing agent for silver ion, and a binder, said layer having therein a merocyanine spectral sensitizing dye represented by the formula:

• ##STR8## wherein R represents a saturated or unsaturated aliphatic group, X represents O, S or Se, R.sub.1 represents an alkyl, aryl, 2-thienyl or 2-furyl group, and Y represents a 1 to 3 carbon atom-containing alkylene group.