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I have been converting several images that are larger than 2 MiB to be less than or equal to 2 MiB to make them compatible with SE's Imgur instance, which has a 2 MiB file size limit. This is because of an upcoming Imgur TOS change that could cause various non-SE Imgur images to be deleted (link leads to the relevant Meta SE post). In most cases, simply converting the images to JPG format with a file size of 2 MiB or slightly less is enough for SE Imgur to accept them without reducing the resolution.

However, for some very high-resolution images, I have noticed that SE Imgur will reduce the image resolution to approximately half of its original resolution if it is 789 KiB or larger; but it will not if it is 760 KiB or smaller. Since I have to target a very low file size of ≤ 760 KiB instead of ≤ 2 MiB to avoid halving the resolution, this requires lowering the image quality significantly.

The images that I have encountered this issue with are 5400 × 1398, 5500 × 5300, and 6093 × 5090 JPG images. Due to the limited sample size, I am not sure if it is simply a matter of file size.

Sample images — notice that if you try to convert these images to be ≤ 2 MiB and then upload the resulting image to SE Imgur, it will reduce their resolutions as I described above:

Makyen, in a comment, adds that a similar, possibly related issue affects PNGs:

SE's Imgur converts images in at least some other formats to JPG when the file size exceeds the 760 KiB mentioned here in the question. For images which are in a lossless format, this can make a striking difference in quality. For example, larger screen captures in PNG format will be auto-converted to JPG, which makes them harder to read and, potentially, introduces artifacts that may significantly alter the question or answers.

Since PNG is a lossless image format, converting a PNG image to JPG, a lossy image format, defeats the purpose of using PNGs in the first place.

To help users prepare their image files so that they are not reduced in resolution when uploaded to SE Imgur, I would like to ask:

What are the factors that cause SE's Imgur instance to reduce the resolution of uploaded images?

Specifically:

  1. What is the file size threshold that triggers SE Imgur to reduce to a lower resolution?
  2. What is the resolution threshold that triggers SE Imgur to reduce to a lower resolution?
  3. By how much does SE Imgur reduce the resolution and/or quality of images that exceed these thresholds?
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  • I think this applies somehow Non-animated images over 1MB for anonymous uploads and 5MB for account holders will be lossily compressed. This compression reduces filesize while maintaining quality.
    – rene
    Commented May 9, 2023 at 9:02
  • @rene I'm not sure if that article applies to SE Imgur. It mentions things that don't seem to apply to SE Imgur (different supported file types and file size limits), and it doesn't say anything about reducing image resolution.
    – galacticninja
    Commented May 9, 2023 at 9:24
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    Yeah, it only confirms that imgur does apply converts in various ways to uploaded images , we just need to know the exact specs for the SE instance.
    – rene
    Commented May 9, 2023 at 9:32
  • 3
    In addition to the conversions above, SE's Imgur converts images in at least some other formats to JPG when the file size exceeds the 760 KiB mentioned here in the question. For images which are in a lossless format, this can make a striking difference in quality. For example, larger screen captures in PNG format will be auto-converted to JPG, which makes them harder to read and, potentially, introduces artifacts that may significantly alter the question or answers.
    – Makyen
    Commented May 9, 2023 at 14:32
  • To avoid misunderstanding, in short the question's author wants to know what are the limits for size (in bytes), resolution (in pixels), and depth (in bpp) that trigger automatic compression and resizing of images on SEImgur? Also, what are the compression and resizing rates of the image?
    – Augusto Vasques
    Commented May 10, 2023 at 8:40
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    @AugustoVasques Yes. Basically, I'm asking for information that can help users prepare their image files so that they are not reduced in resolution or quality when uploaded to SE Imgur.
    – galacticninja
    Commented May 10, 2023 at 10:23
  • FYI: Some uploaded images get aggressively compressed; why and when?
    – Franck Dernoncourt
    Commented Nov 28, 2023 at 10:37

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