One of my favourite things in amateur radio is to work modes that are quite niche. This was especially the case after I had made my custom audio interface PCB for use with my 2M and HF rigs. I had lots of fun sending all sorts of beeps and boops over it. This came during a period when there was lots of experimentation in OARC with lots of different modes that people had never used before, as well as coinciding with the appearance at the time of the shiny new #rx-captures channel - meant for audio clips and data captures (such as RTTY output or weather satellite images).
A popular reception target for those new to amateur radio and HF data in general are the various weather fax transmissions that are sent from various locations around the world. Traditionally something like FLDigi is used to receive these images. However, one thing people may not realise is that WEFAX (in a couple of specific submodes) aren't just receive-only in the software. Like all modes in FLDigi they can be *transmitted* also. However, it isn't QUITE as easy to do as you'd think. Therefore, this guide is meant to give some brief guidance on topics such as image formats and resolution in order to help steer people towards experimentation with this mode from an amateur radio context.
If you've looked into WEFAX reception before you may know that there are various different formats. Usually this relates to lines per minute. As you'd expect a lower line speed results in a potentially higher quality image, more resilient to noise. FLDigi allows you to select from a few different speeds, but 120 LPM will be absolutely fine and matches most (if not all) of the actual weather broadcast stations that you will hear on the bands. Regardless of line speed chosen it seems that the resulting image is always 1809 pixels wide, so it is important to make sure that your image is this wide otherwise FLDigi will fill in the rest with white space (which you may be ok with!). The height of the image is up to you. Longer images take longer to send and will put more stress on your rig. The exercise of choosing a power output for sending an image is left to the reader - the resilience of your finals being the limiting factor.
Of course, there's no real restriction on WHAT you send. I usually make some templates in something that people often make fun of, but which is actually a pretty decent package for simple graphic design: PowerPoint. No, really. You can set up templates with text boxes that you can easily change for things like a callsign, RST, and other notes that you want to send. These slides can be exported out, but they will end up being a 16:9 ratio, most probably 1280 by 720 or similar (TODO: See if you can set up a custom slide size for this!).
So, you'll need to resize your slide to make sure it matches the required 1809 pixels wide. Make sure that when you do this that the aspect ratio stays OK (ie, if you widen the slide you should also be vertically extending the slide). If any further vertical trimming is required to make the image shorter for sending then now is the time to do it.
In terms of filetype FLDigi will accept PNG, BMP, and JPEG. I'd strongly advise to keep things uncompressed so that you have nice, clean lines - BMP is probably best. Experiment if you want! One thing that needs noting is that FLDigi, despite supporting BMP, doesn't give that as an option in the file picker - you'll need to select All Files from the filetype drop-down box first.
Assuming you have CAT control all sorted this is pretty easy. Load FLDigi and select “Weather Fax Image TX” from the View menu. When the new window appears check your LPM is how you want it and then click load and select your image using the file picker. You'll see your image in the window, along with any whitespace to the right if you didn't make it 1809 wide.
Then hit TX B/W or Color as appropriate to send your image. I have never tried colour, it always seems greyed out.