Which one is correct ?

1. If you say "In the past", the sentence should be "Past Tense", and
2. You should not have used "just" because "just" me "present/now", and
3. "... or not" is a question or query, it is not a statement; either you found (=feel) that the picture beautiful or you found(=feel) that the picture not beautiful. It is illogical to write this sentence.
4. "find out" = discover, understand, get to know; it means you are being told or become understanding about "the picture". But "beautiful" is a subjective (personal view/feeling) about "the picture", perhaps you've used the wrong phrase/verb.

So none of the "choices" is correct.

The "maybe" correct answer is:

In the past, I found that the picture was beautiful.

2013-10-28 10:35:36 補充：
Correction/Also can be: In the past, I found the picture was beautiful.

The two "maybe" sentence has slightly different expression but generally mean the same: I though it was a beautiful in the past.

''I just find that the picture beautiful or not'' is correct because...

1. ''In the past, I just find out that the picture beautiful or not''：
it says in the past so it must be past tence, but it says ''I just find'' it is not past tence so it is wrong.

2. ''I just found out that the picture beautiful or not''
it didn't says any thing like ''yesterbay'' ''in the past'' ''last week'' so it should be present tence but it says ''I just found'' it is past tence so it is wrong too.

3. ''I just found that the picture beautiful or not''
Same as no.2.it didn't says any thing like ''yesterbay'' ''in the past'' ''last week'' so it should be present tence but it says ''I just found'' it is past tence so it is wrong.

'just" can be used in the sentence:

" In the past, I just found (that) the picture was beautiful"

When "just" is used as an adverb, it also means "恰當地", "正確地"

eg. The article just mentioned the current situation.

這文章 恰當地/ 正確地 敘述當今的形勢。

In the past, I just found whether the picture is beautiful or not.

文法上是這樣～
意思上仍有點怪～