"we may require the originals of the awarded photographs to be submitted by the relevant participants, with a short side of minimum 2200 pixels and bit depth of 300 dpi."

and

" Contest photographs must be saved in the jpg/jpeg format, 150-300 dpi and 7-12 compression quality, with a short edge of minimum 1920 pixels and a long edge of maximum 3200 pixels, and the size of the files should not be less than 2 Mb and more than 4 Mb."

You are not the only one that does not understand the terminology in these statements. Those who wrote it don't know what they are saying, either.

• 300 dpi is not a bit depth.
• Since file size of a compressed image is highly dependent upon image contents, it might not be possible for some images with the size requirements listed to also comply with the file size requirements. An image with very few details and a minimal number of different colors might well be less than 2 MB even at 3200x2400 pixels and using the highest quality compression setting. Likewise, an image with a lot of different details and many color variations could well be more than 4 MB even when sized to 2560x1920 and only using moderate JPEG compression.

With digital images, the most relevant number is the number of pixels. You can change the "dpi" number (more properly called ppi, for 'pixels per inch' - but then the EXIF specification propagates this error so we can't be too hard on the contest organizers for that) to whatever you wish and it won't change the actual pixels of a digital image, it just changes a number in the metadata.

It sounds like the contest organizers want images that are no more than 3200 pixels on the longer edge of the photo and no less than 1920 pixels along the shorter edge. This means any image with an aspect ratio greater than 1.6667:1 won't qualify. (To meet the 1920 pixel short side minimum, the long side would need to exceed 3200 pixels.)

If your camera shoots in 3:2 aspect ratio (most "full frame" and APS-C cameras), the smallest you can submit your images would be 2880x1920 pixels. The largest you could submit would be 3200x2133 pixels.

If your camera shoots in 4:3 aspect ratio (Four-Thirds and Micro Four-Thirds cameras as well as many compacts and "point and shoot" cameras), the smallest you could submit would be 2560x1920 pixels. The largest you could submit would be 3200x2400 pixels.

Since the organizers reserve the right to see the original images before they were resized and compressed to fit the submission criteria, you need to start with original images that are at least 2000 pixels in the short edge.

How do I make my image comply with the requirements?

If you have a photo that is larger than the acceptable dimensions, in pixels, outlined by the contest requirements you'll need to resize it to fit them.

width 5184 height 3456 bit depth 24 size 6.55 MB (6,878,542.00 bytes)

If you resize your 5184x3456 image to 3192x2128 and use the highest compression quality setting available, your image will almost certainly be smaller than the 4 MB limit, since you're reducing the total number of pixels by a factor of 2.64. (6.55 MB divided by 2.63 is about 2.5 MB.)

Ideally you'll export a 3192x2128 pixel version¹ from the original raw file and use the minimal amount of compression needed to meet the file size limit of 4 MB. You control compression by using the "JPEG quality" setting of most image editing programs. Since they don't specify which specific image editing application they're referencing with the "7-12 compression quality" setting, we'll assume "12" is the highest quality setting with the least amount of compression. Be sure to specify 300 dpi/ppi in your export settings so that the contest organizers are happy that a number that doesn't really mean anything when viewing images digitally meets their requirement.

¹ Thanks to @MooseBoys for pointing out that using multiples of 16 when scaling down an image will give an integer number of macroblocks. Many video codecs use 16x16 macro blocks. Most still image formats, such as jpeg, can still use 8x8 MCU (macro) blocks, so using multiples of 8 also works well. In this case, the largest image size divisible by 8 on both sides of a 3:2 aspect ratio image that meets the pixels per side requirements of the contest is also divisible by 16.