What is a socially acceptable way of getting people interested in Signal? I installed it and use it for messaging (Ive never been able to actually receive an mms with it). But I have not one contact on signal, so it's not doing much for me at this point. My daily work contacts don't seem like a prime target user base.

You're going to have a hard time convincing non-technical people / people who don't acutely care about privacy to use it in my experience. I generally try to shift people who are not in these demographics over to WhatsApp as that has a large user base and implements the signal protocol for end to end encryption. It's not a "safe" as signal but it's far better than something like messenger.

I can't even send pictures on Signal.

There is a camera icon for taking a picture. Otherwise you need to use the attachment function for sending a picture.

I've tried everything AFAIK. The pictures appear inside the chat area but with a red cross on their upper right corner indicating that sending failed.

After you take or attach a picture to the Signal chat you haven't sent it yet; you can write a caption for the image or send it without. The red cross if for removing the image if you change your mind. At least, this is what popped up in my mind immediately after your description.