I’m building a forum site. Should I only allow text based posts like IH?

FYI, you can use markdown and include images.

Initially, you sure can do that. As the users increase and you engage with in discussions, you will find out yourself if people actually need them or not. Maybe setup a poll for users?

Let's back up - what is your differentiation with for instance Covers https://www.covers.com/forum/nba-betting-22/nba-champs-103434196 I feel like debating allowing images and video is getting into the weeds.

Do you have people lined up on some platform already? Like reddit, FB, or elsewhere? If you do not, start there and find people first. If you already have some people somewhere you can gauge what is missing in the current platform and fix those with your own custom solution. Without feedback you are pretty much just guessing blindly.

you should do what your community and members want... what do they want?

I'll provide an argument for images, if only because the thread seems mostly in favor of text-only. The medium is the message. Images, which in your case could include screenshots of betting spreadsheets and outcomes, infographics (either OC or third party), and memes (building a community vocabulary around shared interests) could bring high-effort content to your platform that people perceive as valuable. However, low-quality spam posts, unrelated images, and more could pop up. The question is, how do you incentivize posting great images that are perceived as high-value content? Some options:

1. Institute a reputation system where users of a certain tenure or popularity unlock access to image posts.
2. Organize the user interface such that making an image post is relatively more difficult than making a text post, so users will only go through the effort of posting images if their image is high-effort content. Ways to do this include only accepting and rendering links rather than accepting image uploads yourself or only making a captcha necessary to post images, but not text.
3. Alter your formula for ranking to make it somewhat more difficult, but not impossible, for images to rise to the top of the forum unless they are legitimately and substantially more popular than competing text posts.

I run a pure text based problem validation platform, where even the submissions are only text - https://needgap.com which is gaining enough traction to achieve my goals.

I also run other text based websites (which aren't forums) like my startup coaching website - https://hitstartup.com(It has occasional image posts, but with complete transcription) & curated list of startup tools - https://startuptoolchain.com.

My suggestion from the experience would be, if your target audience are no-frills, no-BS, not addicted to limited attention content, bibliophiles, cautious about where they put their time; then a text only forum can work out great and also be capital efficient. Case in point.

But if your audience spend much of their time on TikTok, Instragram; then text based forum is the last thing they will use.

Personally I prefer text only, at least for the comments. Make sure you protect yourself against script injection!

I think that will be a good idea to get you started. I had a conversation with few people regarding the same topic and the overwhelming feeling was that these type of sites that allows only text are very useful and popular for people who want to have a serious conversation about any particular thing (sports and results, in your case).
However. it would not be sufficient for me to come to any judgement on the basis of a few comments. so, you should probably start asking for feedback on this topic once you have setup the forum.