there's not a darn thing wrong with this picture, judgeing from what I can see from that angle:
In this picture
you do have a chair seat. It looks like you are pushing yourself back into the cantle with your feet. So, rather than riding DOWN into your stirrup, you are pushing them OUT and away from you.
The length, in my eye, is fine. But, you probalby have a somewhat locked knee.
I like the excersize of using your toes to draw in the sand. But, before you do that, try this:
sit in the saddle with horse standing quietly. If horse is not calm enough to sit still while you do this, then get a person to hold the bridle.
Bring your feet out of the stirrups and up in front of you so that they rest on the horse's shoulders, heels right up as high as you can get them , right on the horse's shoulders. try to still sit up as straight and proud as you can, even with your legs hiked up like this. You should feel your seatbones pressing very hard and directly down intot the saddle. Sitting up nice and straight, looking ahead, lower your legs til they hang straight down. NOW, try drawing with your toe if you like. the way your seatbones are now is where they should be all the time.
Now, just lift up the front of your foot until you can hook it onto the stirrup. keep letting your leg hang down.
once you find the stirrup, try to keep the contact with the stirrup just about what is needed to keep your foot from slipping off. and never, ever lock your knee, It must be able to flex so it can absorb the hrose's movement.