How to convert single image to video in infinite loop and set duration of 15 seconds using FFMPEG
ffmpeg -loop 1 -i https://thumbs.dreamstime.com/z/cartoon-weather-images-16836392.jpg -t 15 output.mp4 

FFprobe result:

ffprobe version 3.4.8-0ubuntu0.2 Copyright (c) 2007-2020 the FFmpeg  
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'output.mp4':   Metadata:  
    major_brand     : isom  
    minor_version   : 512  
    compatible_brands: isomiso2avc1mp41  
    encoder         : Lavf57.83.100   Duration: 00:00:15.00, start: 0.000000, bitrate: 188 kb/s  
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc), 1182x1300 [SAR 1:1 DAR 591:650], 186 kb/s, 25 fps, 25  
tbr, 12800 tbn, 50 tbc (default)  
    Metadata:  
      handler_name    : VideoHandler