While working with a member who was having trouble posting iPhone pictures from his Mac computer to this site, I had occasion to research this issue. Here is what I learned.
iPhones, like other cameras, used to store photos in JPEG format. However, in 2017, iPhones began storing photos as HEIF (High Efficiency Image Format) files with a related HEIC format extension. HEIF/HEIC allows photos to be compressed more than JPEG does, which allows more efficient use of storage space, and it also adds some image processing improvements that are mostly of importance to professional photographers.
However, you can't upload HEIC images to this site, or any site, or social media sites, you can't print an HEIC image, and HEIC images will not open in any browser including Apple's own Safari.
So, what Apple did soon after introducing HEIF in 2017 was to enable automatic, on-the-fly conversion of HEIC images to JPEGs whenever the HEIC image is exported from an iPhone to Android phones and, presumably, when uploaded to internet sites. Therefore, you should be able to upload iPhone photos directly from your iPhone to this site.
However, if your iPhone photos end up on your Mac computer as HEIC images (by whatever means Apple software does that), you probably can't upload those photos to this site because Xenforo can't accept HEIC format images. What you have to do is to first convert your HEIC photos to JPEGs on your Mac, using a free online format converter such as this one:
cloudconvert.com
A second work-around, if you don't care whether your phone stores photos as HEIF/HEIC, is to change the settings in your iPhone so that it will always save photos as JPEGs. You do this by going into Settings>Camera>Formats and switch to the Most Compatible setting. After you do this, all your iPhone photos will be JPEGs and you should have no issues posting those photos from your iPhone or Mac computers onto internet sites such as this one.
One source of my info was this article:
www.adobe.com
If anyone has any different experiences or thinks any of this is wrong or incomplete or whatever, please feel free to comment.
iPhones, like other cameras, used to store photos in JPEG format. However, in 2017, iPhones began storing photos as HEIF (High Efficiency Image Format) files with a related HEIC format extension. HEIF/HEIC allows photos to be compressed more than JPEG does, which allows more efficient use of storage space, and it also adds some image processing improvements that are mostly of importance to professional photographers.
However, you can't upload HEIC images to this site, or any site, or social media sites, you can't print an HEIC image, and HEIC images will not open in any browser including Apple's own Safari.
So, what Apple did soon after introducing HEIF in 2017 was to enable automatic, on-the-fly conversion of HEIC images to JPEGs whenever the HEIC image is exported from an iPhone to Android phones and, presumably, when uploaded to internet sites. Therefore, you should be able to upload iPhone photos directly from your iPhone to this site.
However, if your iPhone photos end up on your Mac computer as HEIC images (by whatever means Apple software does that), you probably can't upload those photos to this site because Xenforo can't accept HEIC format images. What you have to do is to first convert your HEIC photos to JPEGs on your Mac, using a free online format converter such as this one:
HEIC to JPG | CloudConvert
>HEIC to JPG Converter - CloudConvert is a free & fast online file conversion service.

A second work-around, if you don't care whether your phone stores photos as HEIF/HEIC, is to change the settings in your iPhone so that it will always save photos as JPEGs. You do this by going into Settings>Camera>Formats and switch to the Most Compatible setting. After you do this, all your iPhone photos will be JPEGs and you should have no issues posting those photos from your iPhone or Mac computers onto internet sites such as this one.
One source of my info was this article:
HEIF file: What it is and how to open it | Adobe
HEIF is fast becoming the darling of device storage. But what is HEIF, and can it rival JPEG when it comes to quality? Find out more in our guide.
If anyone has any different experiences or thinks any of this is wrong or incomplete or whatever, please feel free to comment.