Hello, great mod and all but it's not compatible with Optifine 1.8.9 H6 version as the signs are invisible. Also when trying on an smp server I go on it does not work. I place the sign down and no picture. This is with optifine version H5 which works with your mod.

https://gyazo.com/e7ff6751d730f8066bbc5eeebe4c9990

Thanks for reporting.
I have a Minecraft 1.8.9 and Forge 11.15.1.1722 and SignPicture 2.2.2 and Optifine U6 tried to experiment with SMP, it was not able to reproduce the situation.
Even Forge 11.15.1.1808 also, could not be

Optifine settings and, because it might affect the other Mod, or sorry to trouble you, but you please examine the Mod to compete again?

私はMinecraft 1.8.9とSignPicture 2.2.2とOptifine U6をSMPでいろいろ試してみましたが、その状況を再現することができませんでした。
Optifineの設定や、ほかのModの影響かもしれませんので、お手数ですがもう一度競合するModを調べてもらえませんか？

https://gyazo.com/e39e65f4bb1a37f48664d16fa5905711

Okay I tested on a local server I created and it works fine. Kotaserver is a spigot server so that could be the issue here.

I've checked in Kotaserver. I found NoCheatPlus installed in server. I'll try to improve it not to be detected in the NoCheatPlus.

https://github.com/NoCheatPlus/NoCheatPlus/blob/4e91a9d5fcdc140954bfcd82f04ff237b1e4bb34/NCPCore/src/main/java/fr/neatmonster/nocheatplus/checks/blockplace/AutoSign.java

Ok thanks for logging on and taking a look yourself with help from kotarobo_san.

SignPicture2.3.0 Released!
Since the implementation of the Anti-AutoSign measures I think that it can be installed even KotaServer.

It works! You're the best man. Thank you.

Please, go enjoy this mod.
Have a nice day! xD