It is blood from the neck wound. Sheila had to put her arm out in order to push the trigger and that's how the blood got there.
Incase you missed it. I posted CCTV footage the other day of a man committing suicide with a rifle. He lands in the same position as Sheila with the gun in same position also. For some daft reason it was removed as 'inappropriate' despite the forum having a library full of even more gory and graphic content. 
Sorry you believe it was 'daft' to remove a CCTV video of a young man shooting himself to death but that is just a matter of judgement.
It is true it showed the rifle falling onto his body in a similar position to the rifle on Sheila's body.
I don't think anyone would object if you put up a link to the video as long as you included a warning.
I do not agree with you that the blood trails in the photo you posted up compare with those on Sheila's arm. I used to believe this was so, in fact I supported Alias and Caroline in a discussion a few years ago however I am now certain that the blood trails on Sheila's arm are from cuts to the skin and nothing to do with her neck wound.
I do agree Sheila may have instinctively put her hand to her neck after the first shot which would account for blood on her hand as mentioned by Vanezis in his first handwritten statement and maybe the bloodied finger marks on her nightie.