How do I convert a series of PNGs into an AVI using Avidemux?
Use ffmpeg instead, e.g.

ffmpeg audio.mp4 -framerate 50 -pattern_type glob -i 3_DucksTakeOff_720p50_CgrLevels_SINC_FILTER_SVTdec05_/'*'.sgi -sws_flags lanczos+print_info -c:a copy -c:v libx264 -preset slow -crf 22 -pix_fmt yuv420p output.mkv

adjust the input file naming as necessary.  ffmpeg can read png just fine.  If there are other PNGs in the same dir that you don't want to include, use
-start_number 090001 -i dir/1I%06d.png  -frames 180.  (Note that it matters what goes before -i input, and what goes after.  Stuff before is input options, stuff after is output options.)

leave out the audio file if you don't have an audio track to mux.

Decrease the CRF to get higher quality at higher bitrate.

Increase the preset to slower or veryslow to get higher quality at similar bitrate but slower encode time.

Leave out the -pix_fmt yuv420p to not force ffmpeg to subsample the chroma after converting from RGB (PNG's color format) to YUV.  (Most hardware players will only have hardware decoding for 8-bit 4:2:0 h.264, unfortunately.  Same for GPU-accelerated decoding on PCs.)

Hmm, I'm not sure whether ffmpeg defaults to the bt.709 or bt.601 color matrix for RGB -> YUV conversion.

If you really need to make an AVI (with a DIVX / MPEG4-ASP video stream, I assume) for compatibility with an old hardware player, then see the guide.  With avidemux, did you make a non-standard AVI, like h.264 in AVI?

I highly recommend against avidemux for video encoding.  Its x264 options dialog is ancient and terrible.