FFMPEG, Extract first frame and hold for 5 seconds
Use

ffmpeg -start_number 501 -framerate 30 -t 60.0 -i input1.%04d.jpg 
       -framerate 30 -t 15.0 -i transition1_%03d.png
       -filter_complex "[0]loop=149:1:0[bgheld5];
                        [1]loop=149:1:0[fgheld5];
                        [bgheld5][fgheld5]overlay=eof_action=pass"  out.mp4


The first frames of both sequences are repeated 149 times, so that there are 150 instances (30 fps x 5s). The 0 at the end of loop is the starting index of the frame(s) to loop. The middle 1 is the number of frames to loop starting at the index in the 3rd argument.