Hi silverfox
It helps a lot if you can make the pictures around 800 pixels wide but it's not essential.
Scroll down on the "post a message" page and click (or tap) the "upload attachment" tab. Then you can use the "choose attachment" button to find the picture on your device.
Click the "add the file" button and wait while it uploads. The screen will re-draw and you will see your file listed. It will be added to your message when it is posted. If you want the picture in the middle of your post you have to position the cursor where you want it to appear and choose"place inline".
You can preview to see how it looks before clicking "submit" to post.
I know what you mean about not having your computing gear with you. It is hard struggling to type an a small tablet device and, I find, it gives you a terrible thirst at this time of day.