I'm getting a bit more used to using Plume, specifically with adding photographs to posts. Here's the steps that I found worked for me and so I'm posting them for anyone else that would like them.
Open a blank document (or email, something where you can add plain text), write your description of a photo into this.
From the Dashboard, scroll down to Your Media, go to the Media Gallery and choose 'upload'. Copy the description text to Plume when you upload your photo. Once you've added the Description Text and uploaded the photo, press send. Then on the refreshed screen with your photo, scroll down and make sure you copy the Markdown text over to your document/email as you add each one. You cannot seem to access this later so it's important you capture it at this step.
Keep doing this to add all photos. Only once this is done, copy everything over to your new article. It takes much longer to go to the media gallery, add a photo, then go to the dashboard to find your draft post and insert it into the text box, then go back to the Media Gallery add another photo etc. As they are in different parts of Plume, it's better if you are adding several photos into a post, to prepare it all together first.
The Description text is for the visually impaired when they click on a photo. But it turns out that you can also use the text from this to help write your blogpost!
The photo above is an example of the Description Text and Markdown code for photos. The yellow is the Description Text and the blue is the Markdown code. You copy all of this into your blogpost. You can edit the Description Text, which is now in effect your blogpost text, within the post, the yellow section. But NOT the Description Text within the Markdown code, the blue section.
There isn't an edit function for the Description Text for photos currently, so if you want to change the text that appears in the Markdown code, you need to upload the photo again, write the next text, etc. This is why keeping a plain text copy is useful, as you can just copy and paste, then edit the bit you want to change before pressing send.
If you wish to add a photo to appear in at the top of your post, like I have done with the daffidols, whilst editing the post, scroll down to the bottom and under 'illustration', choose the Description Text of the photo you would like to appear. This will insert the photo automatically. Obviously, if you want to add a photo not listed, you'll need to upload that first.
Finally, I discovered that Plume is a bit temperamental when it comes to the file size of photos. The way you find out is that after you press send, if the Markdown code hasn't appeared, the photo hasn't uploaded. It wouldn't let me upload a photo that was 878KB, or one too big. It was happy with 1.4MB, so I'm sticking to that. It's not too large for slow internet connections, but large enough that you get reasonable detail within an image.
I hope this is helpful. If I come across any other tricks, I'll update this post.
Oh, the photo of the daffidol is because it's nearly mid winter and it's bright and pretty :)