What do you want the posted message to say / include?
If it's just text, you can use the Yammer - Post action in Flow. Here's an old sample of something I used to create a Yammer message with a Yammer message as the trigger; just change to the Dropbox trigger.
You can effectively add an image by inputting the URL of the image; it might give a preview in the post, though I haven't tested this.
Further down in the example linked above is doing a REST call (the API) using the HTTP action in Flow. In your case, you'd want to use the Pending Attachments API.
Ok, so all that to explain what the people tried to tell you, and to show you that it's possible to do what they suggest, even with limited programming skills. But as a reward for reading this far, I'll tell you what might be a better methodology.
Every Yammer group has an email address, and you can use that to post messages. Emails with attachments in the email will be uploaded and posted as well. So instead of messing with any of of the API stuff, use DropBox as your trigger, and email as your action.
Go to the group you want to post in, and look on the right rail. Near the bottom is a "post to this group by email" link. Click that and then copy the email address shown.
Create your Flow and create your Email action, and that address is the target email address. Craft your message to be sent based on the picture from DropBox (I never tried that, so don't know how). The point is, if you can send an email with that picture as an attachment, you can post it to Yammer.
Let us know if this helps.
