It depends. If it causes anxiety to someone, it is illegal. Pictures of drugs could fall into this category.

> Current law allows for restrictions on threatening or abusive words or behaviour intending or likely to cause harassment, alarm or distress or cause a breach of the peace, sending another any article which is indecent or grossly offensive with an intent to cause distress or anxiety,

https://en.wikipedia.org/wiki/Censorship_in_the_United_Kingd...