When creating a listing for Airbnb you will need to have images ready that are set to a specific rate plan for that type. You will need at least 7 images in JPEG format. These images need to meet the minimum compliance standard of 800x500 px, though it is recommended that the images sizes be around 4200 x 2700 px. This can be done in Home -> Property Details -> Images.

When uploading the images you will see the size of the images on top of the thumbnail. (see here)
1. This image is at the size recommended by Airbnb.
2. This image is definitely larger than 800x500 and is more than acceptable.
3. This image only meets the minimum standard, this will pass the compliance test but may not look as nice as the others.
4. This image does not meet the standard and will therefore fail the compliance test. Anything that will fail the compliance test will state "less than 800x500".

If you have had a professional photographer take them you should not need to worry about the image size.

The next step after uploading the images is to assign them to the rate plan that will be chosen to be merged into your Airbnb listing. (see here)
1. Go to Home -> Rooms & Rates.
2. Select the rate plan that will be merged to Airbnb (if you have more than listing then you will need to follow the subsequent steps for each listing).
3. Click on the images which apply to this rate plan. (Please note: You can not change the order of the images, they will be in the order that they were uploaded in)

If you do not meet compliance you will see an error message when attempting the merging process. (see here)
1. Please ensure the number of images are at least 7 if not this may be one of the reasons that you are receiving this error.
2. The number of high resolution images should be the same number as the number of images assigned to the rate plan. If there is a discrepancy between the number such as the number of images is 7 but the number of high resolution images is 6 then there is one image that does not meet compliance. You will either need to upload a higher resolution version or assign a different image that is of a higher resolution. (Please note: If the number of images is higher than 7 and the number of higher resolution images is lower than that but higher than 7 you will pass the compliance test but you lose that image in the process)