In this panel of manga, 俺 (おれ), a male form of "I", is written as オレ. This is most likely to emphasize the masculine feeling of the word. According to http://www.takase.com/Names/HowToWritePart1.htm, hiragana is associated more with femininity, while katakana is thought to be more masculine.
Interestingly, きれい is also written in katakana, as キレイ. This is most likely for emphasis also, though not for masculine vs. feminine reasons.
Sample 2: プラ, エネルギー, チョコスナック --> Foreign words, familiar symbols, abbreviation
Here we have two recycling symbols, one for paper (next to the circled プラ symbol), and one for plastics. プラ is the abbreviated word for plastic (プラスチック), a foreign word, hence the use of katakana.
エネルギー and チョコスナック are written as such for the same reason.
Sample 3: 女のコ, カワイイ--> Emphasis and ?
On the magazine cover, カワイイ is most likely written this way for emphasis.
What I find strange is that おんな is written in kanji (女), while こ is written in katakana (コ). I'm not sure why the two were written as such instead of both being written in kanji or hiragana.
