I’m guessing that you have frame blending turned on for the layer. If so, turn it off and see if that removes the apparent “dissolve transition”.
If I’m right, then what is happening is that you’ve got a low-frame-rate footage item (your 2fps image sequence) in a composition with a higher frame rate, and the frame blending is interpolating between the frames of your footage item to serve the higher frame rate of the composition.
If you want to eliminate the guesswork, feel free to upload a project that shows this behavior, and I’ll have a look.
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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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